Add a variable to disable modules in the PDK

Not plumbed through yet, but will at least prevent us from losing
track of which modules will not build in the PDK.

Test: builds
Change-Id: I4c1a0e39104b104dc1ff23c4391d5e83b3002d3a
diff --git a/android/variable.go b/android/variable.go
index 03c797f..e692faf 100644
--- a/android/variable.go
+++ b/android/variable.go
@@ -81,6 +81,10 @@
 			Cflags   []string
 			Cppflags []string
 		}
+
+		Pdk struct {
+			Enabled *bool
+		}
 	} `android:"arch_variant"`
 }